One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is the creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High power and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). To this end, a plurality of small-sized secondary batteries (unit cells) is connected in series to each other so as to form a battery module. According to circumstances, the small-sized secondary batteries (unit cells) are connected in series and in parallel to each other so as to form a battery module.
Generally, such a battery module has a structure to protect unit modules, each of which has secondary batteries mounted therein. The structure of the battery module may be various based on the kind of vehicles or installation position of the battery module in the vehicles. One of the structures to effectively fix large-capacity unit modules is based on supporting bars and end plates as shown in FIG. 1.
Referring to FIG. 1, a battery module 100 includes unit modules 10, each of which has secondary batteries mounted therein, a base plate 20, a pair of end plates 30, and supporting bars 40.
The unit modules 10 are stacked at the top of the base plate 20 in a state in which the unit modules 10 are erected vertically. The end plates 30 are disposed in tight contact with the outer sides of the outermost unit modules 10 in a state in which the bottom of each of the end plates 30 is fixed to the base plate 20.
The supporting bars 40 are connected between the upper parts of the end plates 30 so as to interconnect and support the end plates 30.
In the structure in which the unit modules 10 are stacked at the top of the plate-shaped base plate 20 in a tight contact fashion, however, strength of the base plate 20 is low with respect to motion in the direction perpendicular to the base plate 20, i.e. the vertical direction. If the base plate 20 does not have vertical strength, the base plate, the base plate 20 may be greatly deformed due to vertical vibration.
Also, in a case in which the battery module is installed in a trunk of a vehicle, a portion of the base plate is mounted above a region where a spare tire is located due to the layout of the vehicle. That is, the battery module is installed in an asymmetrical structure. If vibration from the vehicle due to a road surface is severe, twisting load is applied to the battery module, and such twisting load is transmitted to the base plate with the result that the base plate may be easily damaged.
Therefore, there is a high necessity for a vertical stack type battery module having a structure to stably maintain the stacked structure of the unit modules and to effectively offsetting vertical vibration and properly disperse bending load even when pressure generated from the unit modules are applied to the base plate.